Default web server

I have a multiserver setup. The control panel is on the sql server. I have 2 web server with mirroring. Everything is working.
But when a client add a website, the server used by default is the sql server (the client can't choose).
In the limits of the client account, the default web server is the correct one.
With the admin rights, when I click on this website, the server is the web server (in grey, I can't change it).
The vhost file for this website is created on the sql server.

Where can I change the default webserver for the client at an other place than in the limits of the client account ?